The production of future generations of submicron VLSI silicon microelectronics and compound semiconductor devices will depend critically on continued advances in process sensing and control. In present-day manufacturing, process yield limitations are brought about by the high sensitivity of layer properties to process conditions, and the inability to control process conditions adequately throughout the process sequence. Present technology relies primarily upon the inference of important process parameters from indirect sensor signals, and upon open-loop control using previous calibration data, together with the (often unfulfilled) hope that process conditions will not drift appreciably, both within a given run and from run-to-run.
As the semiconductor industry moves to increase process throughput, increase wafer diameters, tighten uniformity, and decrease batch sizes (culminating with the modern single wafer rapid thermal processor), the need for real-time, whole-wafer sensing is becoming acute. To ensure both wafer-to-wafer repeatability and also uniformity, wafer sensors must provide extremely accurate multipoint measurements, with absolute accuracies and precision of temperature measurement in the range of ±1° C. The high throughputs that are typical of modern semiconductor processes place additional demands on sensor performance, requiring the capability to track thermal processes with temperature ramp rates up to 50° C. per second.
A variety of sensors have been developed for real-time in-situ measurements of temperature, thickness, and other film properties. By virtue of its speed, sensitivity, and low cost, optical pyrometry has become the most widely adopted in-situ temperature sensor, and has become ubiquitous in commercial single wafer thermal processing system. Pyrometry, though, suffers from two significant practical limitations. Firstly, pyrometric sensors, traditionally employed as single point sensors, become less suitable when intra-wafer uniformity is critical. Secondly, pyrometers require accurate knowledge of the wafer emissivity; they become unreliable or inaccurate when the emissivity is not known, when it varies during processing, or when it varies from wafer to wafer. This situation arises quite often during semiconductor processing because the emissivity depends on the optical properties of the films and substrates, which are themselves temperature dependent. The problem of emissivity variation has been addressed in some applications by incorporating a reflection sensor that provides a real-time measure of the specular emissivity at the desired wavelengths. However, the geometric complications imposed by the reflection optics complicates the emission/reflection technique unsuitable for spatially-resolved whole wafer measurements. The practical barriers to achieving fast and accurate temperature monitoring across large wafers is perhaps the most significant hurdle blocking broader applications of rapid thermal process technology.
A variety of sensors have been developed for real-time in-situ measurements of temperature, thickness, and other film properties. Virtually all practical non-contact wafer temperature sensors are based on pyrometry, the quantitative analysis of temperature from the thermal radiation emitted by the hot surface. In general, the thermal radiance R(v, T) emitted by a hot surface is equal to the product of the wavelength dependent emissivity ε(v,T) and the Planck function P(v,T) associated with the wafer temperature:
                              R          ⁡                      (            v            )                          =                                            ε              ⁡                              (                                  v                  ,                  T                                )                                      ⁢                          P              ⁡                              (                                  v                  ,                  T                                )                                              =                                    ε              ⁡                              (                                  v                  ,                  T                                )                                      ⁢                          hcv°                                                e                                      hcv                                                                  k                        B                                            ⁢                      T                                                                      -                1                                                                        Equation        ⁢                                  ⁢        1            In traditional pyrometry, the temperature is inferred from the radiance using an assumed value for the emissivity. In many situations however this assumption is not valid, and can lead to uncontrolled errors in temperature measurements and associated drifts in processing temperature. Pyrometry, as it is applied in semiconductor processing, has traditionally been limited to single point measurements, with multipoint capability requiring multiple pyrometers. A logical extension of multiple pyrometers is the concept of a focal plane array radiometer.
Significant improvements to pyrometry have been demonstrated by integrating the capability to measure the emissivity in addition to the thermal radiance (see Morrison et al U.S. Pat. No. 4,985,858). The emissivity ε at a particular wavelength and polarization can be related to the measured reflectance and transmittance by invoking conservation of energy: 1=ε+ρ+T, where ρ and T are the reflectance and transmittance at the selected wavelength and polarization. If a wafer is sufficiently absorbing and thick that it is opaque at the selected wavelength, then the emissivity is given simply by ε=1−ρ, and the emissivity can then be computed from a measurement of the reflectance (see P. W. Morrison, Jr., P. R. Solomon, M. A. Serio, R. M. Carangelo, J. R. Markham, Sensors Magazine, 8; J. R. Markham, K. Kinsella, C. R. Brouilette, R. M. Carangelo, M. D. Carangelo, P. E. Best, and P. R. Solomon, Rev. Sci. Instrum., 64, 2515–2522, 1993). Such emissivity tracking temperature measurements have been demonstrated using a variety of single wavelength and spectroscopic hardware, and various systems have been integrated into rapid thermal process equipment (see S. Farquharson, P. Rosenthal, P. Solomon, N. Ravindra, and F. Tong, “Development of a Non-Contact Real-Time Sensor for SiO2 Layer Thickness and Temperature in a Rapid Thermal Oxidation Reactor”, Transient Thermal Processing Techniques in Electronic Materials, Ed. N. M. Ravindra and R. K. Singh, The Minerals, Metals & Materials Society, 1996), PLD equipment (see P. Solomon, S. Liu, J. Haigis, P. Rosenthal and S. Farquharson, “Process Monitoring and Control During Plasma and other processing of Semiconductors” AirForce SBIR Phase II Final Report, WL-TR-95-5016, 1995), and MBE equipment (see S. Farquharson, K. Kinsella, J. Markham, P. Solomon, M. Carangelo, J. Haigis, N. Ravindra, F. Tong, M. Bevan, and G. Westphal, “Real-Time Process Control of Molecular/beam Epitaxial Growth of Mercury Cadmium Telluride Films by Fourier Transform Infrared Spectroscopy,” Transient Thermal Processing Techniques in Electronic Materials, Ed. N. M. Ravindra and R. K. Singh, The Minerals, Metals & Materials Society, 1996); they are also the subject of U.S. Pat. Nos. 4,956,538, 5,156,461, and 5,564,830. Such combined emission/reflection techniques, and reflection-assisted techniques, can have excellent accuracy and precision, provided the samples are specular and opaque; however, they are poorly suited to whole wafer sensing during processing.
More particularly, although the use of reflection-assisted emissivity tracking enables extremely accurate measurement of temperature, in many applications the samples are not sufficiently specular to get a useful reflectance measurement, and geometric considerations often prohibit performing reflectance measurements on a dense point grid from a wafer in a process chamber. Other difficulties arise, in many process situations, from the necessity to get reliable calibration reflectance reference measurements for accurate emissivity tracking. This requires the introduction of a calibration reference of accurately known reflectance into the process chamber at very nearly the same mechanical positioning as the sample to be measured, often a difficult task and a significant practical technical barrier.